The present invention relates to an electro-optical device stack and a method and system for manufacturing the same.
Electro-optical device stacks are layered structures comprising an electro-optical layer capable of transducing electrical energy into optical (photonic) energy and/or vice versa. Typical applications include light emitting devices (LEDs) and photo-voltaic devices (PVDs). Within the field of electro-optical device stacks particular interest goes out to the use of organic layers. Organic layers may offer advantages such as providing a flexible device stack suitable for roll-to-roll (R2R) processing.
In one example, an organic light-emitting diode (OLED) comprises an active layer comprising light emitting organic molecules sandwiched between anode and cathode layers. When a voltage is applied over the active layer, this may result in (re)combination of electrons and holes in the active layer leading to photon emission. To create high efficiency devices, the device stack may comprise additional layers such as a charge injection layer that facilitate the injection of holes and/or electrons into the emissive layer. A charge injection layer may e.g. be formed of a material having a work-function between that of an electrode and that of the electro-optical layer. A typical charge (hole) injection layer may comprise an acidic compound such as PEDOT:PSS.
The inventors have found that efficiency and/or lifetime of electro-optical devices may deteriorate as a result of leakage current through the electro-optical layer. Leakage current may be considered wasted current in that it does not result in photon emission. One cause of leakage current may be the proximity and/or contact between the charge injection layer and the cathode layer. This may occur at breaches in the active layer wherein the active layer is absent and/or wherein the active layer comprises dentures or thinner areas between the charge injection layer and the cathode layer. Breaches may result e.g. from insufficient wetting (coverage) by the active layer on the charge injection layer and/or due to the formation of aggregates (bumps) in the charge injection layer that may stick partly or fully through the active layer into the cathode layer.
A partial remedy may be to increase the thickness of the active layer. However, this may lead to increased operating voltage and/or reduced power efficiency of the device.
Accordingly, there is a need for an electro-optical device stack having higher performance. In particular there is a need for a method and system that can conveniently produce such a device stack.